1. Field of the Invention
The present invention relates to a zoom lens system and an electronic image pickup apparatus using the same.
2. Description of the Related Art
In recent years, miniaturization of digital still cameras and mounting of image pickup functions on cellular phones have been advanced. In consequence, it is demanded to further miniaturize and thin image pickup lenses. As these image pickup lenses, there are demanded zoom lens systems having a zoom ratio above 2.5 from a wide-angle end to a telephoto end.
As methods for realizing the thin zoom lens systems, there are known a method of bending the optical axis at right angle by disposing a reflecting member in the zoom lens system, and also a method of moving a part of lens units constituting the zoom lens system to the outside of the photographing the optical path.
However, the method of bending an optical axis by use of a reflective member requires a space for bending a light ray and a space for moving the lens unit in order to secure the zoom ratio. These spaces are not eliminated even at a time when an image pickup apparatus such as a digital camera is not used. Therefore, the method is disadvantageous to decrease of a volume of the image pickup apparatus when unused. Moreover, layout in the image pickup apparatus is limited due to the bent optical axis.
On the other hand, in the method of moving a part of the lens units to the outside of the photographing optical path when unused, a mechanism to moving the lens units is required. Therefore, it is difficult to suppress an influence in a case where the lens unit is eccentric with respect to the optical axis. Since driving mechanism for moving a part of the lens units is required, it is difficult to suppress the volume of the apparatus when unused. This method is also disadvantageous in view of costs.
As a zoom lens system using a collapsible type lens barrel structure in which the miniaturization is achieved, there are known a two-unit zoom lens system having a refractive power layout of a negative-positive type from an object side; a three-unit zoom lens system of a negative-positive-negative type; and a three-unit zoom lens system of a negative-positive-positive type.
Among them, the two-unit zoom lens system of the negative-positive type is advantageous in decreasing a total thickness of a lens frame which directly holds lenses, because the number of the lens units is small. However, in order to secure the zoom ratio, the second lens unit needs to be moved in a region which includes an equal magnification position of the second lens unit during magnification change. In this case, when the second lens unit is moved to perform focusing, the movement direction of the second lens unit during focusing from infinity to a short distance is reversed in a magnification state before and after the equal magnification position of the second lens unit. Further, when the zoom lens system is focused on infinity while the second lens unit is in the equal magnification position, focusing to the short distance cannot be performed by the movement of the second lens unit. That is, the second lens unit cannot be used as a focusing lens unit. In consequence, the first lens unit or the whole zoom lens system needs to be moved for focusing, and the total length of the lens barrel including the focusing mechanism increases. Therefore, this type is disadvantageous in thinning the zoom lens system and securing the zoom ratio.
On the other hand, the zoom lens system of the negative-positive-negative type and the zoom lens system of the negative-positive-positive type are advantageous to the miniaturization in that the increase of the total length can be suppressed by performing focusing by the third lens unit.
Moreover, in the zoom lens system of the negative-positive-positive type, a fluctuation of the exit pupil position during magnification change easily increases. Therefore, a change of an incidence angle of an off-axial ray on a peripheral portion of an image sensor during magnification change easily increases, and a change of an image quality in a peripheral portion of an image easily occurs. A diameter of a front lens (a diameter of the first lens unit) easily increases, an outer diameter of the lens frame increases, and the system is disadvantageous to the miniaturization.
On the other hand, in the zoom lens system of the negative-positive-negative type, the fluctuation of the exit pupil position during magnification change is little, and it is easy to reduce the change of the incidence angle of the off-axial ray on the peripheral portion of the image sensor during magnification change. Since the third lens unit having a negative refractive power has a function of shifting the principal point of the whole zoom lens system toward an object side, it is possible to reduce the total length of the zoom lens system. It is also easy to generally reduce the ray height incident on each lens unit of the zoom lens system for the size of the image pickup surface of the image sensor. This type is especially advantageous in reducing the diameter of the front lens. Therefore, the type is also advantageous to decrease of the diameter of the lens frame.
As a zoom lens system of the negative-positive-negative type, there is known a zoom lens system in which the third lens unit moves to a position closer to the image side in the telephoto end than in the wide-angle end or hardly moves. However, in the zoom lens system of this type, since the third lens unit is positioned close to the image surface in the telephoto end, the height of an off-axial ray increases in the third lens unit, and the diameter of the third lens unit easily increases. In the case where the third lens unit moves for focusing operation, the focusing sensitivity (the movement amount of the image surface position at a time when the focusing lens moves as much as a unit movement amount) easily decreases. Therefore, the third lens unit necessarily has a strong negative power, and it is difficult to suppress generation of aberration in the third lens unit.
As examples of the zoom lens system in which the number of lenses constituting the lens system is small and the third lens unit moves toward the object side during magnification change toward the telephoto end (toward the side having a long focal length), there are known zoom lens systems described in Examples 1 to 3 of Japanese Patent Application Laid-Open No. 2004-294910 and Examples 1 to 3 of Japanese Patent Application Laid-Open No. 2005-77692.